The present invention relates to a system for regulating the idle speed of an internal combustion engine mounted on a vehicle, and more particularly to a regulating system for slowdown of the idle speed due to an increase of electric load such as head lights and other devices.
A conventional regulating system is provided with a feedback control circuit comprising an engine speed sensor, a comparator for comparing the output of the sensor with a reference value and for producing an output, and an actuator responsive to the output of the comparator for actuating a throttle valve of the engine or a control valve provided in a bypass around the throttle valve to regulate the idle speed to a predetermined speed. Japanese Laid Open Patent Application Nos. 54-79324 and 55-160138 disclose systems provided with such control valves. Such a system can not regulate the small variation of idle speed caused by electric load, because the engine speed sensor does not respond to such a small variation of idle speed.
On the other hand a voltage regulator of an alternator mounted on the vehicle causes the alternator to operate to increase the alternator voltage when the alternator voltage tends to drop due to the increase of electric load. Such a regulating operation is performed by controlling the amount of current flowing through the field winding of the alternator. Therefore, if idle speed decreases with the increase of electric load, the current flowing through the field windings.